Unsaturated polyester resins with a novel structure are prepared from monomeric hydroxyalkyl esters of aromatic dicarboxylic acids. The diesters are prepared from the reaction of diacids with alkylene oxides. These resins are produced in less time and are equivalent in performance relative to standard polyesters.
Unsaturated polyesters are prepared in two fashions; single stage esterifications, where all the ingredients are added initially, and a two-stage cook, where the slow reacting saturated diacid is esterified first before adding the unsaturated diacid. With the first method, random copolymerizations occur with phthalic anhydride and block esterifications, with the unsaturated groups internal, occur with isophthalic or terephthalic acid. The two-stage cook produces block polymers with the unsaturation at the ends of the polymer chain. The latter method produces polyesters with superior heat and corrosion resistance. However, it is time consuming.
Mayer, U.S. Pat. No. 3,252,941 devised a method for lowering the time necessary for the reaction, but it suffers from a lowering of the physical properties of the cured plastic.
Miyamoto et al, U.S. Pat. No. 4,306,056 used propylene oxide as an esterification agent for isophthalic acid by direct esterification to yield a high molecular weight isophthalic oligoester. This process is highly complicated and does not reduce the esterification time in the second stage--the slowest section of a two-stage cook.